I was working on a little invention for my wood/coal boiler to put out the fire in case of an electrical outage, but Danfoss beat me to it.

It's called a BVTS thermostatic valve for bio-mass boilers. It can accomplish two purposes. It can add cold water to an overheating boiler, or it can spray water onto a wood/coal fire to extinguish it in the event of a power outage, and it requires no electricity to operate and automatically shuts itself off when the overheating situation is corrected.

For water temperature control, the "valve is used to prevent the water in the bio-mass boiler from overheating. If the water temperature rises, the valve will open and let cold water through the heat exchanger system, thus quickly and efficiently lowering the temperature."

For back-burning protection the "thermostatic valve will operate safely regardless of the ambient temperature and the self-acting technology means that the valve does not reqire electricity or other forms of energy in order to work."

"Thermostatic valves play two very important roles in small, solid fuel boilers. The first is to sprinkle water onto the bio-mass fuel if the burning process moves backwards into the fuel feed screw and magazine The second is to dissipate excess heat so that the water temperature does not exceed 100º C in the boiler."

It has a probe like an aquastat which controls a valve that will allow water to be added to an overheating boiler or even spray onto or into a firebox or coal pipe, to the best of my understanding. I'll post more info when it becomes available.